A known brake system of the general type under consideration is shown in FIG. 1. Referring to FIG. 1, brake system 10 is part of a vehicle having four wheels 12, each of which is provided with a wheel-speed sensor 14, which, via respective electrical lines 18, is electrically connected to a control unit 16. The wheel brakes are provided with respective brake cylinders 20, 22, wherein brake cylinders 22 of the rear axle are designed as combination spring-actuated/diaphragm cylinders. They therefore have a spring-actuator part and a diaphragm part. The spring-actuator part is a parking brake component. The diaphragm part is a service brake component.
The service brake and the parking brake are pneumatically actuated. Braking intent of the vehicle operator is converted by means of a brake actuating device 22 into a modulated brake pressure. This modulated brake pressure is supplied via compressed air lines 24, 26 and 28 to solenoid valves 30 of an anti-lock braking system and via further compressed air lines 32 to brake-cylinders 20 of the front axle. Analogously, the modulated brake pressure is supplied via compressed air lines 34, 36, 38, via a relay valve 40, further via compressed air lines 42 to solenoid valves 44 of the anti-lock braking system and further via compressed air lines 46 to combination spring-actuated/diaphragm cylinders 22 of the rear axle.
Compressed air lines 34, 36, 38, 42 and 46 to combination spring-actuated/diaphragm cylinders 22 of the rear axle form, together with further components, such as solenoid valves 44, relay valve 40 and a compressed air reservoir tank 48, a first brake circuit, referred to as brake circuit I, wherein compressed air reservoir tank 48 is pneumatically in communication via further compressed air lines 50 with brake actuating device 22 as well as relay valve 40. Analogously, compressed air lines 24, 26, 28, 32 as well as solenoid valves 30 form, together with brake cylinders 20, a second brake circuit, referred to as brake circuit II, wherein brake actuating device 22 is in communication via further compressed air lines 52 with a second compressed air reservoir tank 54. Both compressed air reservoir tanks 48, 54 are supplied with compressed air by a compressor.
The parking brake can be actuated by means of a pneumatic switch 56. The spring-actuator part of combination spring-actuated/diaphragm cylinders 22 can be vented by means of switch 56, whereby the parking brake is engaged. The parking brake is released by admission of air to the spring-actuator part. For this purpose, via compressed air lines 58, 60, 62, compressed air can be supplied to combination spring-actuated/diaphragm cylinders 22 and discharged therefrom. Also, for this purpose, compressed air control is exercised, on the one hand, via switch 56 and, on the other hand, via an inverting relay valve 64 as well as a valve 66.
At the same time, inverting relay valve 64 performs a further function. For this purpose it is in communication with the modulated pressure of front-axle brake circuit II via compressed air line 24 and a compressed air line 68, as well as with the modulated pressure of rear axle brake circuit I via compressed air line 34 and a compressed air line 70.
In the event of failure of the first brake circuit, that is, if service braking by means of brake circuit I is no longer possible, inverting relay valve 64 is tasked with activating the parking brake, or, in other words, the spring-actuator part of combination spring-actuated/diaphragm cylinders 22. Even in the event of failure of rear axle brake circuit I, therefore, the rear axle can be braked sufficiently by means of the spring actuators to achieve the necessary braking distance.
In the event of failure or drop of pressure in rear axle brake circuit I, a high modulated pressure of front-axle brake circuit II and a low or zero pressure of rear axle brake circuit I is present at inverting relay valve 64. For this purpose, the inverting relay valve is designed such that, in the event of such a pressure difference between the modulated pressures of the two brake circuits I and II, it lowers the output pressure at inverting relay valve 64, or, in other words, the pressure in compressed air line 60. Thereby, the spring actuators of brake cylinders 22 of the rear axle are vented, so that the parking brake is able to assist or take over the function of the service brake. Even in the event of failure of the rear axle brake circuit, therefore, a specified braking distance can be achieved.